2 Plant Anatomy

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PLANT ANATOMY

The science of the structure of the organized plant body learned by

dissection is called Plant Anatomy (anatomy-dissection). In general,

Plant Anatomy refers to study of internal morphology, pertaining to

different tissues. The subject of this chapter is structure of Angiosperms,

with emphasis on primary tissues.

TISSUE SYSTEM

Plant body in Angiosperms is differentiated into root stem, leaf

and flower. All these parts are made up of different types oftissues containing different cell types. A tissue is a mass of

similar or dissimilar cells performing a common function.

The body of a vascular plant is composed of dermal tissue, Ground tissue

and Vascular tissue.

Dermal Tissue (Skin)

Dermal Tissue is protective in function. Basing on its origin, it isclassified into two types Epidermis and Periderm.


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Epidermis: This is the primary surface tissue of the entire plant.

Epidermal cells are compactly and continuously arranged the continuity

is lost by the presence of Stomatal pores or breaks in the tissue. Covering

the aerial epidermis, cutin (fatty substance) is present as an

impregnation on cell wall. The cuticle can be separated from epidermis.

The epidermis may produce unicellular or multicellular hairy outgrowthsand other appendages. Epidermis provides mechanical protection,

allows gaseous exchange through stomata, restricts transpiration with

cuticle, and is also involved in storage, photosynthesis, secretion,

absorption and perception to stimuli. Stomata are Pores, each guarded

by two guard cells, which control the size of the pore. Cells surrounding

guard cells, but differing from other epidermal cells, are called

subsidiary cells. Guard cells are kidney shaped their cell walls are thick

on the inner surfaces. Guard cells contain many chloroplasts.

Periderm: This is formed during secondary growth replacing primary

epidermis.

Ground Tissue

This is inner to dermal tissue and is composed of simple tissues like

parenchyma.

Vascular tissue

Vascular tissue consists of conducting elements xylem and phloem.

Vascular tissue may be scattered in ground tissue or regularly arranged

forming a ring. In the latter arrangement, ground tissue is differentiated

into cortex (outer to vascular tissue) and pith (inner to vascular tissue).

The ground tissue of leaves is called mesophyll, bound by upper and

lower epidermis.

TISSUE TYPES

In broad sense, tissues are classified as meristematic and permanent

tissues.

Meristem (Meristos divisible)

Initially all embryonic cells of an embryo have the capacity to divide and


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multiply but as the embryo develops into a plant body, this capacity for

division is restricted to certain parts of the plant body called meristems

which are active throughout the life of the plant body (unlike that of an

animal body). When meristematic cells divide, a group of the daughter

cells remain meristematic the other daughter cells called derivatives

differentiate into various tissue elements. Before the occurrence of anycell division, usually cells become enlarged accompanied with addition of

protoplasmic and cell wall material.

Meristematic cells are isodiametric, compactly arranged with dense

cytoplasm, large nucleus, and small vacuoles or without vacuoles. Cell

walls are thin.

Meristems which occur at the apices of stem, root and other branches are

called apical meristems, which bring about primary growth of the plants,

hence also called as primary meristems. In many plants in addition toapical meristems, lateral meristems like vascular cambium, cork

cambium, intercalary meristems are found. Lateral meristems are

arranged parallel to the sides of organs in which they occur.

Intercalary Meristem: This is also a primary meristem, found

inserted between permanent tissues, in the bases of internodes and leaf

sheaths of grasses. Sometimes, as in gynophore of groundnut, xylem maybe present in intercalary meristem. Wherever stem is jointed, elongation

of internodes is due to intercalary meristem. Example: Bamboos. Even

prolonged growth of leaves, flowers and fruits may be regarded as an


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intercalary growth.

Vascular cambium and cork cambium: These are referred to as

secondary meristems because they produce secondary tissues, and

increase the thickness of the plant body. This process is called secondary

growth, seen in dicotyledons and gymnosperms.Permanent or Mature Tissues

Cells derived from meristems gradually change in their structure,

metabolism and chemistry and acquire specialized characters by their

various modes of differentiation. Not all the cells totally differ from the

meristems. Some cells retain the power of division and others cannot

divide. In a strict sense only cells which have lost the power for division

must be regarded as permanent tissues, but in a broad sense, cells

derived from meristem that have acquired a special function like

photosynthesis, secretion, storage are treated as part of matured tissue.

There are different types of mature tissues. Example: Parenchyma,

Collenchyma, Sclerenchyma, Xylem and Phloem.

Types of Mature Tissues

Parenchyma (Para-beside, enchyma-In poured)

Parenchyma is the fundamental tissue of the plant body. It is found in

every part of the plant body like pith and cortex of stem and root,

mesophyll of leaves, flesh of fruits, floral parts and even in xylem and

Phloem. Cells have thin primary walls and polyhedral shapes. The

average number of faces of a polyhedron is 14 and is called as "tetra

decahedron". Cells are compactly arranged or more commonly spaciously

arranged with intercellular spaces as in cortex and pith. Cells possess

dense cytoplasm and are active metabolites.


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Generally cells of parenchyma are involved in storage of starch, sucrose,

protein, water, phenol derivatives, many mineral substances, etc. Other

metabolisms like respiration, protein synthesis etc., are active.

Parenchymatous cells may also perform specialized functions and are

structurally modified. The following are the different types of

parenchyma.

Aerenchyma: Intercellular spaces filled with air, are large in size and

many in number. Cells occupy a smaller area. Though the cells are

smaller, they provide the required strength to the aquatic plants. In

these plants air spaces are common, helping in aeration and buoyancy.

Air spaces are also seen in roots of grasses, petioles of canna, aroids etc.

Chlorenchyma: Cells of photosynthetic parenchyma contain numerous

chloroplasts. These cells are commonly seen in leaves, some times inyoung shoots. Cells of Chlorenchyma are of two types

1) Palisade cells that is elongated and compactly arranged.

2) Spongy cells that are spaciously arranged and irregularly shaped.

Prosenchyma (Pros-to): Cells are elongated, have thick walls and serve as

a supporting tissue. Example: Endosperm cells of seeds.

Armed Parenchyma: Cells are stellate in shape with inward


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projections of cell wall as arms. Example: Pinus leaf Mesophyll.

Collenchyma (Kolla Glue)

Collenchyma is a simple, living tissue. Cell walls are thickened due to

deposition of pectin. Collenchyma is the primary supporting tissue in

stems, leaves and floral parts of dicots, where as in stems and leaves ofmonocots collenchyma is usually absent, (instead, sclerenchyma is

present in monocots).

Collenchyma is usually hypodermal in position. Cells are more elongated

and narrower than parenchyma. Like Parenchyma, collenchyma may also

contain chloroplasts or may regain the thickening. Intercellular spaces

may or may not be present. Three forms of collecnchyma are recognized

based on the types of thickenings

1. Thickening is on the tangential wall lamellar collenchyma.

2. Deposition of pectin is in the corners where several cells meet-

angular collenchyma.

3. Thickenings are around the intercellular spaces lacunar

collenchyma.

Being plastic, collenchyma differs from elastic fibers of sclerenchyma.

Sclerenchyma (Scleros-hard)

Cells of sclerenchyma are thick walled and are usually lignified. The

thickness is due to formation of secondary wall. At least initial the

secondary wall is free from primary wall. At maturity, usually the cells

are devoid of protoplast. The cell wall encloses a cavity lumen. On the

cell wall, pits are usually present (simple type). Commonly sclerenchyma

cells are classified into fibers and sclereids. The primary function ismechanical. (Pits-thin areas without secondary wall material

deposition).


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Fibers: These are usually long, spindle shaped structures, with tapering

or blunt ends. Longest fiber is seen in Boehmeria nivea (55 cms). Theyare arranged in groups. Secondary thickening may account for 90% of

the area of the cell the lumen is narrow. Fibers are grouped into xylary

fibers and extraxylary fibers. Xylary fibers, also called wood fibers are

parts of xylem and are longest among xylem elements. Extraxylary fibers

are classified as Bast or phloem fibers, cortical fibers and perivascular

fibers (peripheral to the vascular bundles). These are lignified or non

lignified.

On account of their elasticity, fibers enable the plant body to withstand

various strains. Commercial fibers like jute, flax, ramie are extraxylary

fibers.

Sclereids: These are shorter than fibers. Sclereids occur singly or in

groups. Sclereids are commonly found in fruit wall, seed coat, epidermal

scales, and occasionally found in cortex, pith, mesophyll and petiole of

submerged aquatics.

There are many types of sclereids

1. Asterosclereids are star shaped

2. Macrosclereids are similar to palisade cells

3. Osteosclereids are bone like that are enlarged at their ends

4. Brachysclereids are isodiametric like parenchyma

Vascular Tissue


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This tissue is also called complex tissue. It is heterogeneous in nature

with different types of cells. The chief elements are xylem, phloem and

pericycle.

Xylem (Wood)

Xylem is the water conducting tissue. It consists of living cells likeparenchyma and dead cells like tracheary elements. Fibers are also

present. Xylem elements differentiating from an apical meristem

constitute primary xylem where as secondary xylem is differentiated from

vascular cambium. Xylem is the most preserved tissue in fossils, due to

the development of lignified secondary walls.

Tracheary elements (trachea rough): These are of two types 1).

Trachea or vessels 2) Tracheids, Vessels are limited in their growth, are

joined end to end to form continuous tubular structures with

perforations in their cross walls. Water and minerals are efficiently

conducted through these perforations. Vessels are present in most of the

Angiosperms and also in certain lower plants like Gnetum, Marsilea and

Selaginella.

Tracheids are generally elongated and non-perforated. They have only

pit pairs, at the regions of union with other tracheids. Conduction of

water and minerals is not as efficient as in vessels. Tracheids are found


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in Pteridophytes, most of the Gymnosperms and a few Angiosperms.

Secondary thickenings in tracheary elements are deposited in various

forms-1) as rings, called annular. 2) as continuous helices, called helical

or spiral, 3) as net work called reticulate, 4) as transverse nets called

scalariform, 5) as extensive thickenings except in the region of pits,called pitted.

Primary Xylem: This is differentiated into Protoxylem and Metaxylem,

denoting the xylem that appears first. In stems, protoxylem is found

nearest to central axis (endarch xylem) and in roots away from centre

(exarch xylem). The protoxylem elements have annular, spiral and

sometimes reticulate secondary thickenings. Fibers are absent in

protoxylem. Metaxylem appears after protoxylem. Metaxylem has only

pitted secondary walls. Metaxylem is more complex than protoxylem and

its elements are wider.

Secondary Xylem: The vascular cambium (intrafascicular cambium

interfascicular cambium) producing secondary xylem consists of fusiform

and ray initials. Here also the secondary xylem is also of two-system-1)

Axial or vertical system with living and nonliving cells and 2) ray or

horizontal system with living cells. When compared with primary xylem,

secondary xylem is more complex and shows orderly development. Pitted

and scalariform secondary thickenings are developed.

Phloem (Phloos Bark)

Phloem is the food conducting tissue. It is composed of sieve elements,

parenchyma cells, fibers and sclereids. Phloem differentiated from

procambium is called primary phloem secondary phloem is initiated

from vascular cambium.


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When xylem and phloem elements are found in the same line the

arrangement is called conjoint if phloem is only external to xylem it iscalled collateral as in many stems if phloem is both external and internal

to xylem it is called bicollateral as in leaves and a few stems if xylem

completely surrounds phloem it is called amphivasal if phloem

surrounds xylem it is called amphicribal if xylem and phloem alternate

with each other as in roots, it is called radial.

Sieve Elements: These are classified into sieve cells and sieve tubemembers. Sieve cells are commonly long, slender, and taper at both

ends. These cells overlapping, many sieve areas are seen. Sieve areas


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have cluster of pores through which adjacent sieve cells are

interconnected by protoplasmic strands. Surrounding the pores,

carbohydrate Callose material is found in a sectional view. Sieve cells

are found in Pteridophytes and Gymnosperms. Sieve tube members are

tubular, placed in long series and have specialized sieve areas called

sieve plates occurring at end walls. These sieve plates are ofteninclined. In addition to sieve plates, there may be also sieve areas on

lateral walls of sieve tube members. In sieve plate, sieve areas, having

large pores are seen. Initially sieve tube members contain uninucleated

protoplast. Gradually the nucleus and its associated endoplasmic

reticulum become disorganized. Mitochondria may lose their inner

membranes. Cytoplasm is usually parietal. In the centre of the cell a

mixture of vascular sap and disorganized cytoplasmic matter is seen.

Tonoplast is not found surrounding the vacuole. Studies sieve contents

show that they are under positive pressure of 30 atm and velocity of

conduction is 100 cm/ hour. The transported material is usually sucrose

which is more than 20% in concentration.

Sieve tube member is associated with one or more specialized

parenchyma cells called companion cells are seen only in Angiosperms.

Both sieve tube member and companion cell are derived from the same

meristematic cell. Often, companion cells of sieve tube members form

longitudinal series. Companion cell is nucleated. Probably these cells

provide energy for food conduction.

Primary Phloem: This is classified into Protophloem and

Metaphloem. In protophloem, sieve tube members are without

companion cells. Sieve tubes function for a brief period and soon they

get crushed by the surrounding pressure. The crushed cells may

disappear. Metaphloem tissue survives for a longer period. Its elements

are longer and wider. Usually fibers are absent in dicotyledons, where as

in monocotyledons and herbaceous dicots, parenchyma cells are absent.

Secondary phloem: Similar to secondary xylem, there are two systems

of arrangements in secondary phloem 1) Axial system producing sieve

elements, phloem parenchyma and phloem fibers. 2) Transverse system

producing ray parenchyma cells.


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Pericycle (A Circle All Around)

Pericycle is the region, consisting of one few layers of cells, found

external to central cylinder (Stele). Example: Roots and Dicot stems.

Pericycle may be composed of parenchyma cells or sclerenchyma cells or

both. Lateral branches and phellogen may arise from pericycle.

Summary

Tissue: a group of cells performing a particular function is called

tissue. Basing on the types of cell structure and function, tissues are

classified into Meristematic, Simple and Complex tissues.

Meristematic Tissues: These are actively dividing cells, which are

isodiametric in shape, rich in cytoplasm, with small vacuoles or no

vacuoles and cells are actively metabolic. Depending upon the position

they are classified into apical meristem (at the tips of the stem and

roots), lateral meristem (vascular and cork cambium), intercalary

meristem (at the basal portion of internodes in Graminae members).

Apical meristem is responsible for the primary growth, ex., growth of the

plant in length lateral meristem is responsible for secondary growth and

intercalary meristem is responsible for the elongation of the internodes

in Bamboo plants etc.

Simple Tissues: Tissues contain cells of similar structure, function and

have common origin.

Method of study: Stem cuttings are put in a macerating fluid and

allowed for some time meanwhile some think T.S. are taken and stained

with saffranin and mounted on a slide. The treated stem pieces are taken

on the slide, washed, teased and mounted on the slide for microscopic

observation.

Parenchyma: Important living tissue found abundantly in cortex, pith

of the stem, root and also in the mesophyll tissue of leaves. Depending

upon the structure, content and function they are differently named like

storage tissue, chlorenchyma, spongy parenchyma, ground tissue,

aerenchyma etc. Cells are thin walled, rich in cytoplasm, with small or a


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large central vacuole, actively metabolic and loosely arranged with small

or large intercellular spaces.

Collenchyma: Important Living mechanical tissue, found mostly just

below the epidermis of stem, similar to that of parenchyma but with

difference in the cell thickenings. Here cells are larger, compactlyarranged and cell wall at the angles of the meeting of cells is

characteristically thickened by pectin deposition.

Sclerenchymatous Fibers: These are true mechanical tissues either

dead or living. Two types of cells are found a) Sclerenchyma fibers b)

Sclereids.

Sclerenchymatous Fibers: They are dead cells, long with tapering

ends, with uniform thickening, leaving a narrow lumen. They are found

below the epidermis, cortex, around vascular bundles and so on.

Sclereids: They are found in fruit or seed walls and also in the flesh of

apples and pineapple. The cells may be living or dead, but short and

variously shaped cell wall is heavily thickened with many pit lines

unevenly distributed.

Complex Tissues: The cells that are grouped here are of various types

in their structure, shape and function, and have different origin, but

together they perform a common function Example: vascular tissues,

secretory tissues etc.

Vascular Tissues: 1) XYLEM: It is a water conducting tissue, made up

of tracheids, trachea, xylem fibers and xylem parenchyma of which the

first three are dead cells and the last one is living.

TRACHEIDS: have tapering ends with secondary thickenings of annular,

spiral, scalariform and reticulate types and cell walls are pitted.

TRACHEA: are vessels with the tapering ends and their transverse walls

are disintegrated, so that cells are placed end to end to form a kind of

continuous channels for conducting water and minerals.

XYLEM FIBRES: are similar to Sclerenchymatous fibers. All the above

three tissues are dead and by having thick secondary walls they act as


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mechanical tissues.

XYLEM PARENCHYMA: even though its cell walls are thick and pitted it

is living and perform some visit functions probably active transportation

etc.

3)

PHLOEM: This tissue is solely responsible for the transportation ofthe food material from the source i.e., leaves and storage organs to

the regions of need. It consists of sieve tubes, companion cells,

phloem parenchyma (all living) and phloem fiber (dead cells).

SIEVE TUBE CELLS : are long large cells with end walls perforated,

but cushioned with callus material and the end plates are called

sieve plates, through which large number of cytoplasmic strands

pass through. The cell has a large central vacuole with thin

peripheral protoplasm. The most remarkable feature is the

complete absence of nucleus. Most of the food material is

conducted through this sieve tube in the form of sucrose.

4) COMPANION CELLS: are small cells, living, found and pressed to

sieve tubes. Cells are rich in cytoplasm and actively metabolic. The

function is probably providing energy for transportation of food

materials by active process. PHLOEM PARENCHYMA: is similar to

parenchyma, and found around sieve tubes and companion cells

PHLOEM FIBRE OR BAST FIBRES: are mostly dead cells with

considerable and varied type of secondary thickenings.

PRIMARY GROWTH

The primary growth of a plant body is brought about by the activity of

primary meristems, particularly by apical meristem. Details about

primary growth occurring in stems, leaves and roots, may be studied by

observing a transverse structure is differentiated into outer epidermis,

the inner ground tissue and vascular tissue.

Dicotyledonous stem: Example: Sunflower (Helianthus annus).

The transverse section is almost circular in outline.

Epidermis: This is the outermost layer, uniseriate, covered with thin


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cuticle, and bears a few stomata and many multicellular trichomes.

Ground Tissue: This fundamental tissue is differentiated into outer

cortex and central pith.

Cortex: This is many layered and differentiated into hypodermis andinner cortex. Hypodermis is found below the epidermis, and is

constituted with angular collenchyma which is 3-5 layered.


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The inner part of cortex is constituted with chlorenchyma and

parenchyma large numbers of intercellular spaces are present between

these cells. The inner most cortical layer is wavy in outline, rich in starchgrains, with barrel shaped cells and is called endodermoid layer.

(Endodermis is generally absent in stem endodermoid layer differs from

endodermis in lacking casparian thickenings).


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Pericycle: This is the outermost region of vascular tissue. This consists

of alternately arranged patches of sclerenchyma and parenchyma. The

sclerenchyma is also known as hard bast fibers. Since it is found external

to vascular bundle, the patch of sclerenchyma is also called bundle cap.

Vascular tissue: This consists of many vascular bundles arranged

regularly in the form of broken ring (Eustele). In each vascular bundlethe xylem and phloem elements are in the same line the presence of one

to few stripped intrafascicular cambium is found in between xylem and

phloem the cambial cells are thin walled and are almost rectangular

phloem is only external to xylem: the protoxylem is nearest to centre.

The vascular bundles are conjoint, collateral, endarch, open and wedge

shaped.

Pith or Medulla: This is inner to vascular tissue and occupies the

central region. It is composed of cells of parenchyma, with intercellular


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spaces. Extensions of these cells into the areas in between vascular

bundles are called medullary rays.

Monocotyledonous stem: Example: Zea mays (Maize). The outline is

circular.

Epidermis: This is the uniseriate, outermost layer and is covered with

thick cuticle. Trichomes are absent.

Ground tissue: The ground tissue is not differentiated into cortex and

pith, since there is scattered arrangement of vascular bundles. Even

endodermoid layer and pericycle are also not differentiated. Hypodermis

constitutes the exterior ground tissue, and is composed of 2 3 layers ofsclerenchyma. The major part of the ground tissue is parenchymatous.

Vascular tissues: This consists of numerous, oval shaped vascular

bundles scattered throughout the ground tissue, hence the arrangement

of vascular tissue is called atactostele. Peripheral vascular bundles are

small-sized but many in numbers. The central vascular bundles are large

sized but a few in numbers in all the vascular bundles the xylem andphloem are in the same line cambium is absent phloem is only external

to xylem protoxylem faces the centre. Hence the vascular bundles are


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described as conjoint, collateral, endarch and closed. Each vascular

bundle is surrounded by a bundle sheath of sclerenchyma, which is

prominent at the base and tip of vascular bundle. Xylem comprises

usually 2 oppositely arranged Metaxylem elements (pitted, facing

periphery) and a few protoxylem elements (facing the centre of stem) in a

linear row. If an imaginary line is drawn connecting the two Metaxylemelements with that of protoxylem, xylem appears Y shaped. Usually the

lower protoxylem elements break down and form a lysogenous cavity.

Above the metaphloem, a small band of obliterated protophloem is seen.

Metaphloem consists of sieve tubes and companion cells but not phloem

parenchyma.

Comparison between the stems of Dicot and Monocot:

Dicot stem

Ex: Sunflower

Monocot stem

Ex : Maize

1. Epidermal tricho me s Pre se nt

multicellular

Absent

2. Hypodermis Collenchymatous Sclerenchymatous

3. Ground tissue Differentiated into

cortex and Pith

Undifferentiated

4. Endodermoid layer

and

Peri-cycle

Present Absent

5. Stele Eustele Atactostele

6. Vascular bundles Many open,

wedge- shaped

Numerous,

closed, oval-

shaped

7. Lysigenous cavity Absent Present

8. Bundle cap Present Absent

9. Bundle sheath Absent Present

10. Secondary Growth Occurs later Does not occur

Dicotyledonous root: Example: Lablab purpureus (Beans)


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Epiblema: The external protective layer is called Epiblema or Piliferouslayer (the term epidermis is generally not applied to roots). It is

uniseriate. Cells

are thin walled. Some of these cells extend into unicellular root hairs.

Cortex: The region inner to epiblema is called cortex and it is

homogenous. The cortex is composed of thin walled, spaciously arranged


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parenchyma cells that constitute many layers. With increase in age, the

exterior cortical region becomes suberized and dead. This exterior cortex

is known as exodermis. The innermost cortical layer is called

endodermis. Its cells are barrel-shaped and are closely packed. Many

cells get suberized on their radial and inner walls. These thickenings are

called casparian stripes. Such endodermal cells may become totallysuberized on all wall surfaces and are called stone cells. These cells do

not allow conduction. The non-suberized endodermal cells are called

passage cells. Inner to endodermis, one or two layers of parenchyma are

present, constituting pericycle.

Stele: Xylem and phloem patches are in equal number and alternate

with each other. Xylem or phloem patches are usually four in number.

In Xylem, the protoxylem elements face pericycle between xylem and

phloem elements, parenchyma cells are present constituting conjunctive

tissue, which later becomes a secondary meristem. Hence vascular

bundles are described as radial, exarch and open.

Pith is very small, with closely packed parenchyma cells.

Monocotyledonous root: Example: Canna

Outline almost circular.

The nature of epiblema, exodermis, cortex, endodermis and pericycle is

similar to that of Dicot root. However in cortex, large air spaces of

schizogenous origin may be seen. The nature and arrangement of

vascular bundles are also similar to that of Dicot root. But vascular

bundles are usually more than six in number. The Parenchyma present

in between xylem and phlegm is called conjunctive tissue, with thin cell

walls. This tissue does not become meristematic and thus differs from

Dicot root. Pith is relatively large and parenchymatous. The

parenchyma of the

Pith and conjunctive tissue, become lignified and Sclerenchymatous. The

vascular bundles are described as polyarch, radial, exarch and closed. An

endogenous lateral branch may be seen developing from pericycle and

passing through cortex.

Comparison between the roots of Dicot and Monocot:


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Dicot Root

Ex: Beans

Monocot Root

Ex : Canna

1. Vascular

bundles

Diarch to hexarch, open Polyarch closed.

2. Pith Small or absent Relatively large

3. Cambium Conjunctive tissuebecomes meristematic

and results in

secondary growth

Conjunctive tissuedoes not become a

meristem and

complete absence of

secondary growth.

4. Peric ycle It may g ive ri se t o c ork

cambium and lateral

branches

It may give rise to

only lateral

branches.

Comparison between the structures of stem and root:

Stem Root

1. Cuticle Present, thick / thin Usually absent

2 . T ri ch om es W he n p re se nt , i s m ul ti -

cellular or unicellular

on the Epidermis.

Epiblema hairs are

unicellular

3. Stomata Present Absent

4. Hypodermis Either col le nchymatous

or sclerenchymatous.

Not distinct from

inner cortex.

5. Cortex When present, is

heterogeneous, with

collenchyma,

parenchyma andsclerenchyma.

Homogenous with

cells of parenchyma.

6 . E nd od er mi s U su al ly a bs en t

endodermoid layer may

be present

Generally present

7. Pericycle Two to few

layered/absent

One layered

8. Vascularbundles

Many, conjoint,collateral or

bicollateral, regularly

2-12, radial, exarch,form a broken ring.


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arranged forming a

broken ring or scattered

in ground tissue,

endarch.

9. Pith Well marked and large

or not differentiated

Relatively small or

absent.

1 0. B ra nc hi ng E xo ge no us , or ig in at in g

from cortex

Endogenous, arising

form pericycle.

Dicotyledonous leaf: Example:Mangifera (Mango)

The leaf is horizontal and dorsiventrally differentiated (upper = ventral,

lower = dorsal).

Epidermis: The two surfaces are bounded by their respective epidermal

layers. The epidermal cells are barrel-shaped, compactly arranged

upper epidermis is covered with thick cuticle and lacks stomata lower

epidermis is light green, covered with thin cuticle and is interrupted by

stomata.

Mesophyll: This is the ground tissue. It is differentiated into upper

palisade and lower spongy parenchyma.


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Palisade parenchyma is found immediately below the upper epidermis, 2

to 3 layered, with compactly arranged tubular cells, rich in parietal

chloroplasts.

Spongy parenchyma is found above the lower epidermis these cells are

varied in shape and sizes, very loosely arranged enclosing air spacessome of which open into stomata. Chloroplasts are parietal in the

parenchyma cells.

Vascular tissue: Vascular bundles vary in size each bundle is conjoint,

collateral and closed. The vascular bundle is covered by a bundle sheath

of parenchyma cells. An extension of bundle sheath is seen above and

below the vascular bundle, reaching the epidermal layers. Phloem is

towards lower epidermis xylem is towards upper epidermis, with

metaxylem facing phloem. Fibers are absent in both xylem and phloem.

The xylem and phloem elements are conspicuous only in large vascular

bundles.

Monocotyledonous leaf Example: Maize.

The leaf is Isobilateral in nature and is vertically oriented.

Epidermis: This is uniseriate, with barrel-shaped, compactly arranged

cells and is covered with thick cuticle.

Stomata are found on both upper and lower epidermal layers hence it is

called amphistomatic (more on the lower epidermis). Though the leaf is


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referred to as isobilateral, it is only in upper epidermis, a few large,

empty and colorless bulliform or motor cells are present. During dry

weather, these motor cells help the leaf to role over, due to the changes

in turgidity. This rolling of leaf reduces the rate of stomatal

transpiration.

Mesophyll: There is no differentiation of mesophyll into spongy and

palisade parenchyma. All the cells of chlorenchyma are alike,

isodiametric, almost compactly arranged with numerous parietal

chloroplasts.

Vascular tissue: The vascular bundles are numerous, arranged in

parallel series (venation is palmate-parallel), conjoint, collateral and

closed. Phloem is towards lower epidermis. Each vascular bundle is

surrounded by chlorenchymatous bundle this sheath also serves for

temporary storage of starch. A few vascular bundles are larger in size,

with more amount of xylem and phloem and with large bundle sheath

cells. A patch of sclerenchyma is present above and below the large sized

vascular bundles.

Comparison between the leaf structures of Dicot and Monocot.

Dicot Root

Ex: Mango

Monocot Root

Ex : Maize

1. Orientation of

leaf

Horizontal,

dorsiventrally

differentiated

Vertical, isobilateral

2. Stomata Usually on lower

epidermis

On both epidermal

layers

3. Cuticle Thick on upper

epidermis

Thick on both

epidermal layers

4. Motor cells Absent Present in upper

epidermis

5 . M es op hy ll D if fe re nt ia te d i nt o

upper palisade

parenchyma and lower

spongy parenchyma

No differentiation of

mesophyll

6. Vascular All of them are not seen

Vascular bundles


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bundles in parallel series and

are supported by

bundle sheath

extension

are in parallel series

and are supported

by sclerenchyma

patches

Secondary Growth in Dicot StemIncrease in lateral thickening is called secondary thickening or secondary

growth. This growth occurs in stem and root parts of shrubs and trees in

Dicotyledons. This development takes place in stele (intrastelar

secondary growth) and also in cortex (extra stellar secondary growth).

Intrastelar Secondary Growth: In each vascular bundle between

xylem and phloem a strip o intra-fascicular (within vascular bundle)

cambium is present. Between vascular bundles, medullary rays

composed of parenchyma are found. A few parenchyma cells of these

rays adjacent to intra-fascicular cambium dedifferentiate into

meristematic cells and constitute inter-fascicular cambium (in between

vascular bundle). Both inter-and intrafascicular cambiums join to form

a complete ring of cambium called vascular cambium, which initiates

intrastelar secondary growth.

The vascular cambial cells continuously divide and form numerous

daughter cells. These derivatives produced inward towards pith mature

into secondary xylem elements, while those produced outward toward

cortex differentiate into secondary phloem elements. The amount of

secondary xylem tissue produced is much larger than that of secondary

phloem tissue. Due to the formation of secondary xylem and phloem, the

primary xylem and phloem become separated. The primary phloem is

pushed outward and is finally crushed due to internal pressure. The

primary xylem is pushed towards the centre of the axis however primary

xylem elements are not crushed because the cell walls are lignified.

Between bundles of secondary tissues parenchyma cells constituting

vascular rays are found.

Extrastelar secondary growth: To keep pace with the expanding

central stelar region and to maintain the area volume equilibrium

between the stele and cortex, epidermis and pericycle, with parenchyma


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or collenchyma cells acquires secondary meristematic property by

dedifferentiation and is called phellogen or cork cambium. This

cambium is similar to vascular cambium in cutting off cells on its outer

and inner faces. The outer derivatives gradually lose their protoplast and

become dead their cell walls become suberized. These cells constitute

cork or phellem. These inner derivatives remain living and differentiateinto parenchyma of collenchyma. This newly formed inner zone is called

secondary cortex or phelloderm. The phelloderm, phellogen and phellem

are the primary covering layer i.e., epidermis gets ruptured. The newly

formed periderm becomes the protective layer.

Lenticels: the external phellem or cork is impermeable to conduction or

exchange of gases however it is not a continuous zone a few gaps are

seen in cork. In these gaps, a mass of parenchyma cells (complementary

tissue) is seen. This structure is called lenticel. This helps in exchange

of gases, between the atmosphere and inner cortical cells. Through

lenticels also, transpiration occurs and is called lenticular transpiration


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(loss of water is insignificant).

Bark: In cortex of trees, generally many phellogens arise, each one

producing outer phellem and inner phelloderm. If such a process takes

place, whatever tissues lie outer to the inner most phellogen, become

suberized and dead. This structure is called bark, which is of two types 1) continuous bark 2) scaly bark or discontinuous bark. Bark acts as

insulation. It is thicker in temperate tree forms.

Growth rings: In temperate woody plants and in a few tropical trees,

the activity of vascular cambium is seasonal, but not uniform throughout

the year. This is evident only in the region of secondary xylem. The

cambium is less active in autumn, producing few small xylem elements

that are having small lumen. This is called late wood or autumn wood.

In spring the cambium is more active producing large number of large

sized xylem elements that are less dense with large lumen. This is called

spring wood or early wood. The difference in secondary xylem correlated

to the cyclic seasonal changes result in the formation of alternate rings

of autumn wood and spring wood. Thus in each year two zones of

secondary xylem are formed and both can be easily demarcated. These

two zones constitute an annual or growth ring. By counting the number

of annular rings, it is possible to know the age of a plant. The

availability of auxin may be regulating the activity cambium.

Wood: The elements of secondary xylem constitute wood. An active

wood, containing water and stores food substances is called sap wood or

alburnum. It is peripheral. Its living parenchyma cells store and

translocate food. Sap wood is soft and light in color.


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With

advance in age, the central axis of secondary xylem becomes inactive,losing water and stored food material. It becomes infiltrated with

various organic chemicals like oil and ergastic substances (Passive

byproducts of metabolism). This region becomes hard in texture and

dark in color and is called Heart wood or Duraman. This zone loses the

function of conduction of nutrients. Thus an active wood in addition to

conduction of water also conducts nutrients.

2 Plant Anatomy

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